Dry forming systems, and in particular air laying systems, in which the fiber orientation is randomly distributed in the plane of the web or fabric, are used now commercially in the manufacture of a variety of liquid absorbent products. Generally, in the air forming process the fibers, which may be cellulosic, synthetic, or a combination of both, are suspended in a gas stream (e.g., air) and then conveyed to a forming screen where the fibers are formed or condensed into a web. However, the resulting web lacks integrity, and therefore one of several techniques is used to bond the fibers and thereby stabilize the structure. The fabric products produced are soft, flexible and porous, and are suitable for a number of commercial products, particularly disposable products. The fiber content, at least to a large extent, used in many of these products is hydrophilic or can be rendered hydrophilic, and therefore the products are especially useful as liquid absorbent products, such as disposable diapers, incontinent pads, wipes, feminine napkins, and filtration materials.
In the conventional manufacture of air laid products, the loose web condensed on the forming screen is typically stabilized by mechanical, thermal, or chemical means. Mechanical or thermal means have been used extensively, and usually require fiber entanglement or fiber bonding. Chemical bonding utilizes a solvent or adhesive, and U.S. Pat. No. 3,575,749 to Kroyer discloses bonding the fibrous layer with a latex binder, which may be applied to one or both sides of the web. It has long been recognized, however, that chemical bonding with a latex binder is disadvantageous for use in certain products in that the binder impairs the wipe dry characteristic of the web in that the web has poor retention of the liquid.
More recently, water insoluble hydrogels or superabsorbent materials, typically in particle form, have been incorporated into the fibrous web in order to increase the absorptive capacity of the web. These hydrogels have an absorptive capacity for water and body fluids far exceeding that of the hydrophilic fiber, e.g. wood pulp fiber used in the web, and in fact are capable of absorbing twenty times or more their own weight of water and retain this fluid under pressure. Hydrogel particles have two serious limitations, however, that initially have militated against their acceptance in absorptive fibrous products. One limitation is the hydrogel particles, if not used properly, exhibit gel blocking, a phenomenon that inhibits liquid transmission to the interior; and, secondly, when hydrogel particles are incorporated into a web or fabric, the particles tend to migrate or sift and as a consequence, during manufacture, storage or use, the particles migrate from the useful part of the product or can be lost or cause dust. These limitations needed first to be resolved before hydrogels could be utilized to any appreciable extent in liquid absorbent products, e. g. disposable diapers. The prior art is replete with different mechanical means to achieve this objective, typically involving a roller compression or densification step. In U.S. Pat. No. 4,610,678 to Weisman et al., the web bearing the superabsorbent is densified by calender rolls, and the patent expressly avoids the use of solvents or other liquids which, as stated at column 2 of the patent, can impair the absorptive capability of the hydrogel and impart stiffness to the product. In U.S. Pat. No. 4,260,443 to Lindsay et al., the web containing hydrogel particles is embossed to provide land areas of adhesion and thereby confine the hydrogel to the desired areas. A disposable absorbent is disclosed in U.S. Pat. No. 4,500,315 to Pieniak et al., comprising a fibrous layered structure with the superabsorbent sandwiched between layers, and the composite is then compressed.
Still further, U.S. Pat. No. 4,640,810 to Laursen et al. discloses a system for forming an airlaid web, and proposes that a superabsorbent material may be incorporated into the web. The airlaid product is useful for disposable diapers, feminine napkins, underpads, and liquid filters. The patent also discloses in a general manner that the fibrous layer can be bonded to impart integrity to the web, and although alternate bonding methods are suggested, the patent describes using only heated embossing rollers in conjunction with the inventive process and apparatus. The background section of the Laursen et al. patent discusses the Kroyer patent, supra, but hastens to conclude that stabilizing the web or fabric with latex for absorbent products has numerous manufacturing disadvantages and drawbacks, and places complete emphasis on stabilizing the web with heated rolls and embossing.
In fact, the use of superabsorbent materials and latex in the same web or batt is considered counterproductive in that the latex, which in conventional practice is applied as an aqueous emulsion or dispersion or solution, will block the powder and impair or completely destroy its effectiveness. For example, U.S. Pat. No. 4,551,191 to Kock et al., which discloses an airlaid web containing superabsorbent material, states expressly that the absorption rate and capacity of the superabsorbent particles are adversely affected by certain process conditions such as those involving adhesives. This disadvantage was shown to be overcome by the teachings in U.S. Pat. No. 4,600,462 to Watt, which discloses that impairment of the water absorbency of the fiber caused by the latex binder can be overcome by substantially coating the fibers of the formed web with an aqueous solution of a water soluble hydrophile, preferably after the application of the binder. According to Watt, the hydrophile interacts with the binding agent within the matrix to overcome water repellency.
This invention has therefore as its purpose to overcome the deficiencies of the prior art, and to provide a soft, absorbent structure that exhibits high integrity and relatively low bulk.